Filter membrane clogging problem of prefabricated vertical drains (PVD) usually presents in dredged clays by vacuum preloading.For four kinds of drainage plate membrane,the physical and mechanical tests were carried out.The environmental scanning electron microscope used to observe membrane pore structure rules.The essential factors of gradient ratio were comparative analyzed by filter clogging test.The filter membrane clogging mechanism was proposed in summary,and the anti-clogging criterion was established.It should be reference for future engineering.